This is a proposal for five years consisting of three projects involving monkeys instrumented for chronic unit recording. All projects rely heavily on the technique of spike-triggered averaging of emg activity to identify neurons that produce postspike facilitation (PSF) of average emg activity. We interpret clear PSF as evidence that the cell is synaptically linked to motoneurons. We refer to cortical neurons that produce PSF as corticomotoneuronal (CM) cells; red nucleus neurons as rubromotoneuronal (RM) cells. Spike-triggered averaging of emg activity is a powerful tool because it enables identification of a cell's target muscles and quantitation of the strength and latency of its output effects on those muscles. Most importantly, this can be done in the awake monkey where relations between firing rate and various aspects of active and passive movement can be investigated. The objective of the first project is to test the hypothesis that following damage to the corticospinal system, rubrospinal neurons undergo changes in their properties that contribute to the recovery of motor function. To test this hypothesis we will compare the postspike effects and discharge properties of RM cells in pyramidal tract sectioned monkeys with those of RM cells in normal monkeys determined under the same conditions. The objective of the second project is twofold: 1) to assess the contribution of the ventral lateral nuclei of the thalamus to the modulation of CM cell discharge during voluntary movement, and 2) to document the magnitude and temporal characteristics of PSF from neurons that have a minimum disynaptic linkage to motoneurons. This data will then provide and estimate of the extent to which polysynaptic linkages could contribute to the PSF from CM and RM cells. The third project focuses on a fundamental question concerning the output organization of descending systems. This question is as follows. Do some cells of descending systems facilitate functional synergies of muscles that mediate specific, coordinated movements? This proposal stems from our recent finding that a subset of RM cells cofacilitate flexor and extensor muscles. These cells are ideally suited to mediate tasks that require co-contraction of wrist flexor and extensor muscles, such as power grip. Experiments in this proposal will test the extent to which the output effects of RM cells related to specific phases of an arm reach and food taking task are turned to facilitate the muscles that are coactivated during particular phases of the task.